Orochimaru has always been one of the better villains in the series. However, I don't see him as the final villain. It is pretty hard to top Edo Madara, Obito, and the Juubi.
Orochimaru resurrected some of the most powerful people in the past (the four previous Hokage), but that doesn't mean that he could control them. I'm fairly certain that Hashirama and Tobirama both know how to release themselves. Some people say that they would have broken free during their fight with Sarutobi, but they weren't as powerful then. Orochimaru and Kabuto have since perfected the technique, which makes them harder to control and makes it possible for them to break free (like Hashirama did before).
At this point, if Orochimaru turned evil, Naruto, Sasuke, or the shinobi alliance would squash him in a heartbeat.